This invention relates to a process for preparing latexes for use in cationic asphalt emulsion or in other formulations such as composites or coatings, where a cationic latex could be used. Typically, latexes are prepared as anionic in character and their aqueous applications are therefore limited to uses in compatible systems i.e., anionic systems, unless the latex can be converted to, or prepared as, a cationic latex.
Preparing a cationic latex from a conventional emulsion polymerization process without requiring modification of present equipment would be ideal. If the latex is prepared as cationic during the polymerization of the monomers to polymer, modification of the present anionic systems would be required to accommodate the cationic polymerization system. However, if adding stabilizing agents will convert the polymer latex from the anionic form to the cationic form, such modification of polymerization systems would not be necessary.
One use for the cationic latex is in cationic asphalt emulsions. The asphalt emulsion containing latex is used in the construction and repair of roads, pavements, soil stabilization and the like. Typically, cationic asphalt emulsions are prepared from the combination of asphalt and an emulsifier solution of soap, latex, and acid. The asphalt emulsion, containing latex, is cured to an asphalt residuum and the residuum exhibits improvement in properties such as elasticity and ductility over residuum that does not contain latex.
One of the first known methods of incorporating latex polymers into asphalt residuum consisted of mixing the latexes with an emulsified asphalt. If the asphalt emulsion is anionic, the anionic latex will be compatible with the asphalt emulsion because the emulsion polymerization of butadiene and styrene to form synthetic rubber is most commonly carried out in an anionic system. Blending a latex elastomer with an anionic asphalt emulsion to impart improvement in properties such as elasticity and ductility of the asphalt residuum after the asphalt emulsion has cured, is practiced in the art. However, if the asphalt emulsion is cationic, the conventional anionic latex will not be compatible with the asphalt emulsion.
Cationic asphalt emulsifiers are more versatile compared to anionic emulsifiers because cationic emulsifiers result in an asphalt emulsion that can be worked satisfactorily with a broad range of aggregate, especially those aggregates that contain varying amounts of silica. The probable reason for the cationic asphalt emulsion superiority is that silica containing aggregates carry a negative charge and cationic emulsions, which are positively charged, are readily attracted thereto and readily become bound to these aggregate surfaces. The opposite charges facilitate a high demulsifying velocity which advantageously permits rapid reopening of the paved area to traffic.
Various techniques have been used to compatibilize an anionic latex with a cationic asphalt emulsion. A latex which contains nonionic emulsifiers can be combined with a cationic asphalt emulsion. However, the surfactant used to stabilize the latex remains in the asphalt after demulsification and can act as a diluent to the elastomeric properties of the modified asphalt residuum. One such elastomeric property is measured by Percent Torsional Recovery, which is defined by the California Department of Transportation in Torsional Recovery Test Procedure CT332. If a good percent torsional recovery is desired in the asphalt residuum, the surfactant may interfere with such desirable properties.
One method to compensate for the interference of surfactant in the asphalt residuum, is by emulsion polymerizing latex monomers in the presence of nonionic emulsifiers and/or amphoteric additives. Nonionic emulsifiers have a latex stabilizing effect and prevent the latex from precipitating or coagulating as it passes through the isoelectric point from anionic to cationic form. Amphoteric additives are also stable in anionic or cationic mediums depending upon their charge, however such additives are inherently unstable at the isoelectric point.
Not all asphalt specifications require the use of latex in the asphalt emulsion. In the typical process of preparing an asphalt emulsion, two individual tanks, one containing asphalt and one containing a mixture of soap and an acid, are combined in a colloid mill to produce the asphalt emulsion. For asphalt emulsions requiring elastomer modification, typically a nonionic latex, polychloroprene, is added to the aqueous solution of soap and acid. The soap and the acid in the aqueous tank will alter the nonionic latex to cationic character thereby preserving the stability of the latex in the emulsifier solution. The latex is added to the aqueous tank and the aqueous tank contents are then combined with the asphalt. However, it would be advantageous to be able to add the latex to the asphalt emulsion directly, that is, after the asphalt is combined with the aqueous solution of soap and acid to form the asphalt emulsion. If the latex could be post-added, the asphalt emulsion could be prepared without the latex and when asphalt specifications require the use of latex in the asphalt emulsion, the latex could be subsequently added without having to prepare a separate latex asphalt emulsion.
A need therefore exists for an economic cationic latex asphalt that will overcome the disadvantages of the prior art and will combine the advantages of a cationic asphalt emulsion and a latex.
Accordingly, the present invention provides a method of converting an anionic latex to cationic form without altering the polymerization system. The invention also provides for a system of preparing asphalt emulsion whereby the latex is stable in the asphalt emulsion and the latex may be added to the asphalt either with the aqueous solution of soap and acid, or the latex may be directly added to the asphalt emulsion.
Increasing the performance of the asphalt emulsion would also be desirable. As discussed above, the percent torsional recovery of a latex in an asphalt emulsion can be undesirably reduced by the modifications of the anionic latex to the cationic form. It would also be desirable, therefore, to prepare a latex which when added as a component of the asphalt emulsion yields a greater percent torsional recovery in the asphalt residuum of the asphalt emulsion. A greater percent torsional recovery is thought to be indicative of greater wear, longevity and chip retention in the repaired pavement.